Hot press forming method

ABSTRACT

A steel plate is heated so that an average temperature becomes Ac3 point or higher, a press forming is performed on the steel plate by use of a mold, and the steel plate is cooled during press forming. A formed product is removed from the mold, and the formed product is cooled further. In the hot press forming method, although a quantity of change in shape of the formed product decreases as a removal temperature of the formed product decreases. When the removal temperature of the formed product is lower than 322° C., no more improvement on an accuracy of the formed product can be realized, and a time during which the mold is occupied exclusively to cool the formed product becomes long. When the removal temperature of the formed product is 510° C. or lower, the rate of change of the quantity of change in shape with change in temperature is increased drastically, whereby an increase in accuracy of the formed product can be realized. Consequently, the removal temperature of the formed product is set to a temperature range from 322° C. or higher to 510° C. or lower.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hot press forming method for press forming a heated steel plate and more particularly to an improvement in technique for removing a formed steel plate from a mold.

2. Related Art

In a press forming method, for example, a mold 100 shown in FIGS. 4A to 4C is used. In the mold 100, as is shown in FIG. 4A, when an upper mold 110 is lowered towards a lower die 120 after a workpiece W is placed on an upper surface of the lower die 120, a pad 112 of the upper mold 110 is brought into abutment with the workpiece W and then presses the workpiece W to thereby effect a form shaping on the steel plate.

Next, when the upper mold 110 is lowered further following the form shaping, the abutment state of the pad 112 with the workpiece W is maintained. Then, a trimming blade 115 of a trimming portion 111 of the upper mold 110 is brought into abutment with the workpiece W, and a cutting operation of the workpiece W by the trimming blade 15 is effected. A piercing blade 114 of the upper mold 110 is brought into abutment with the workpiece W, and a cutting operation of the workpiece W by the piercing blade 114 is effected. Reference numeral 113 denotes an elastic member which supports the pad 112, reference numeral 121 a trimming blade provided on the lower die 120, and reference numeral 122 a hole portion into which a portion of the workpiece W which is cut by the piercing blade 114 falls.

In the hot press forming method, the steel plate which is heated at a high temperature of 800° C. or higher is used as the workpiece, and the press forming is performed by the mold 100, and the press formed plate steel is cooled. A hot press forming method like the one described above is promising as a method for obtaining a highly strong press formed product. Specifically, making the steel plate highly strong is realized by changing the structure of the steel plate from an austenite structure (resulting when the steel plate is heated) to a martensite structure (executing so-called quenching) at the same time as press forming by making use of the press mold 100 as a heat removal material (a refrigerant) for removing heat that the steel plate possesses.

In the method above, it is generally necessary that the cooling speed of the steel plate is equal to or faster than the quenching limit speed (27° C./s) and that the temperature of the formed product is reduced to 100 to 200° C. just after it is removed from the press mold to complete the quenching.

However, since the mold needs to be occupied exclusively for cooling the formed product in the way described above until the cooling is completed, compared with a cold press forming method, the hot press forming method requires a longer cycle time, and hence, the productivity thereof becomes worse than that of the cold press forming method. Then, to shorten the cycle time, JP-A-2005-288528 proposes that the formed product be removed from the press mold prior to cooling for quenching in a separate step. However, as this occurs, since the formed product is removed from the press mold prior to cooling, there exists a serious risk of occurrence of a deformation in the shape of the formed product in association with cooling, making it difficult to ensure the accuracy of the formed product with respect to shape.

With a view to solving the problem, deep studies have been made on hot press forming methods, and as a result thereof, it has been found that the accuracy of a formed product depends on the temperature thereof when the formed product is removed from the mold after press forming and that there exists a critical point in the dependency on temperature of the accuracy of the formed product. Thus, the invention has been completed based on these findings.

SUMMARY OF THE INVENTION

One or more embodiments of the invention provide a hot press forming method which can not only shorten a cycle time but also ensure the accuracy of a formed product with respect to shape.

In accordance with one or more embodiments, a hot press forming method may include the steps of: heating a steel plate so that an average temperature of the steel plate becomes Ac3 point or higher; executing a press forming on the steel plate whose average temperature is so set by use of a mold; cooling the steel plate during press forming; removing a formed product from the mold, while the average temperature of the formed product stays within a temperature range from 322° C. or higher to 510° C. or lower; and cooling the formed product further.

The “average temperature” denotes an average value of temperatures of a plurality of locations which are set over a whole area of the formed product.

In the hot press forming method, although the accuracy of the formed product increases as the average temperature of the formed product when it is removed from the mold (hereinafter, referred to as a removal temperature) decreases, the accuracy of the formed product becomes substantially constant when the removal temperature of the formed product becomes lower than 322° C., and no more improvement in accuracy cannot be realized. Moreover, in this case, the time during which the mold is occupied exclusively to cool the formed product becomes long. On the other hand, when the removal temperature is 510° C. or lower, the degree at which the accuracy of the formed product increases with change in temperature increases drastically.

Based on the understanding above, shortening the cycle time is compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower.

The hot press forming method of the invention can adopt various configurations. The degree at which the accuracy of the formed product increases with change in temperature becomes the largest when the removal temperature of the formed product stays in a temperature range from 366° C. or higher to 451° C. or lower. Consequently, a lower limit value for the removal temperature of the formed product is more preferably set to 366° C., and an upper limit of the removal temperature of the formed product is more preferably set to 451° C. or lower.

According to the hot press forming method of the embodiments, shortening the cycle time can be made compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between a temperature of a workpiece (a steel plate) and time in each step of a hot press forming method according to an exemplary embodiment.

FIG. 2 is a graph showing a relationship between quantity of change in shape of a formed product obtained by the hot press forming method according to the exemplary embodiment and removal temperature.

FIGS. 3A and 3B are diagrams explaining dimensions of formed products obtained by the hot press forming method according to the exemplary embodiment.

FIGS. 4A to 4C are side sectional diagrams representing steps of the press forming method.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, the invention will be described in detail by reference to an exemplary embodiment. In the exemplary embodiment, a draw forming is performed on heated steel plates which are introduced into a mold. Average temperatures of formed products when they are removed from the mold (hereinafter, referred to as removal temperatures) are changed formed product by formed product, and accuracies of the formed products are evaluated.

FIG. 1 is a graph showing a relationship between a temperature of a workpiece (a steel plate) and time in each step of a hot press forming method according to the exemplary embodiment. Temperatures in the exemplary embodiment are average values (average temperatures) of temperatures measured at a plurality of points on steel plates by a thermo-viewer. Since temperatures of steel plates cannot be measured during press forming, in FIG. 1, temperatures of steel plates during press forming are obtained from temperatures of formed products measured when press forming is initiated and when press forming is completed on the assumption that the temperatures of the steel plates change linearly during press forming. In FIG. 1, a quenching limit line (a quenching limit speed of 27° C./s) of the steel plate is drawn as well.

In the exemplary embodiment, steel plates (whose carbon content is 0.2 wt % or more) are used as workpieces. Steel plates are firstly heated to 950° C. in a heating oven (a heating step). A material of the steel plate suitable for the invention is an Fe-based material containing, for example, C or C and other elements and including Fe and impurities that cannot be avoided as the remaining portion thereof. This material preferably contains 15 wt % or more C to ensure a required hardness in quenching, and as this occurs, Cr, Mo, Ti and B may be added as required for quenching properties. In addition to the elements above, Si, Mn, P, S and the like may also be added as required.

After the completion of heating, the steel plates are removed from the heating oven, and the steel plate which are still being heated are installed into a mold (an installation step). Following this, the steel plates are press formed by the mold, and the steel plates are cooled (quenching) during press forming (a forming step). This forming step includes a step of cooling the steel plates with the mold held at its bottom dead center (an in-mold restrained cooling step by removing heat by the mold). In this case, quenching is designed to start during press forming.

In the press forming described above, two types of formed products having sectional shapes shown in FIGS. 3A, 3B are obtained by use of two types of molds. FIG. 3A shows a sectional shape of a formed product whose formed height is low, and FIG. 3B shows a sectional shape of a formed product whose formed height is high. Following this, the formed products are removed from the molds to be transferred (a transfer step), and the formed products are cooled (quenching) (a cooling step).

In the exemplary embodiment, a cooling speed during press forming by the mold is set to fall within a range from 50 to 100° C./s, and a removal temperature of formed products is set to fall within a range from 250° C. to 650° C. The graph shown in FIG. 1 shows the results of specific examples of steel plates which are set to fall within the temperature range. In the specific examples, a time from the end of heating of the steel plate to the removal of the steel plate from the heating oven is set to 1 second, a time from the removal of the steel plate from the heating oven to the start of press forming (an installation time) is set to 2.6 seconds, a press forming time is set to 5.4 seconds, a time from the completion of press forming of the steel plate to the start of cooling thereof (a transfer time) is set to 1.7 seconds, and a cooling time is set to 2 seconds. The temperature of the steel plates when press forming is initiated is 850° C. and the temperature of the formed product when press forming is completed is 390° C.

A quantity of change t in shape of a flange portion of the formed product (a different t in height of an edge portion of the flange portion between a shape actually obtained and a normal shape (FIGS. 3A, 3B)) is measured on each formed product after the cooling step. In FIGS. 3A, 3B, a shape indicated by a solid line represents the normal shape (a desired shape), and shape indicated by a broken line (only a left-hand side portion is shown) represents a shape actually obtained. Numeral values in FIG. 3 denote sizes (unit in mm) and angles of portions of sectional shapes of the formed products. The results of measurements on those portions of the formed products with a high formed height and the formed products with a low formed height are shown in Table 2 and FIG. 2.

TABLE 1 Formed Height Low Formed Height High Temperature Accuracy Temperature Accuracy Remarks 261 0 280 0 276 −0.1 296 −0.1 285 0.3 305 0 285 0 306 0.1 286 0.1 322 0.3 Critical Point [1] 331 0.1 339 0 344 0 362 −0.2 353 −0.2 366 0 Critical Point [2] 382 0.2 389 0 385 −0.3 393 0.3 390 0 404 0.4 423 −0.1 412 0.5 431 0.1 434 0.7 440 0.6 438 0.1 451 0 443 0.4 459 0.2 447 0.8 471 0.8 450 1.5 502 1.5 451 1.2 Critical Point [3] 504 0.5 471 1 518 0.9 510 2.15 Critical Point [4] 567 1.45 510 1.1 583 2.3 533 2.6 588 2 541 1.9 600 1.6 541 1.9 606 1.1 545 1.7 622 1.95 658 2.9 630 2.2 658 2.6

FIG. 2 is a graph showing data shown in Table 1. In FIG. 2, a curve l is a curve showing data on the formed products with a high formed height and the curve l is also an approximate curve of maximum values of quantities of change t at individual temperatures. A curve m is a curve showing data on the formed products with a low formed height and the curve m is also an approximate curve of minimum values of quantities of change t at individual temperatures. The curves l, m are obtained so that all the data on the formed products with a high formed height and the formed products with a low formed height are contained between the curves. A broken line N is an image line emphasizing critical points on the curve l.

As is seen from FIG. 2, it is confirmed that although the quantity of change t in shape of the formed product decreases as the average temperature of the formed product when it is removed from the mold (hereinafter, referred to as a removal temperature) decreases, the quantity of change t in shape of the formed product becomes substantially constant when the removal temperature of the formed product becomes lower than 322° C., and no more improvement in accuracy of the formed product cannot be realized. Moreover, in this case, the time during which the mold is occupied exclusively to cool the formed product becomes long. Consequently, it is confirmed that the removal temperature of the formed product needs to be set to 322° C. or higher (Critical Point [1]). On the other hand, it is confirmed that when the removal temperature is 510° C. or lower, the rate of change of the quantity of change t in shape of the formed product with change in temperature increases drastically. Consequently, it is confirmed that the removal temperature of the formed product needs to be set to 510° C. or lower (Critical Point [4]).

Thus, it is confirmed based on the facts above that shortening the cycle time is compatible with ensuring the accuracy of the formed product by setting the removal temperature of the formed product to the temperature range from 322° C. or higher to 510° C. or lower. Since a force which deforms an internal shape of the formed product becomes larger than a resisting force to deformation of the material outside the above temperature range, it is assumed that a change in accuracy of the formed product is induced. In contrast to this, since the relationship between the two forces is reversed, causing the resisting force to deformation of the material to be larger than the force which deforms the internal shape of the formed product within the temperature range, it is considered that the accuracy of the formed product can be ensured.

As is seen from FIG. 2, it is confirmed that the degree at which the accuracy of the formed product increases with change in temperature becomes the largest when the removal temperature of the formed product stays in a temperature range from 366° C. or higher to 451° C. or lower. Consequently, it is confirmed that in order to obtain the advantage described above with good efficiency, a lower limit value for the removal temperature of the formed product is more preferably set to 366° C. (Critical Point [2]), while an upper limit of the removal temperature of the formed product is more preferably set to 451° C. (Critical Point [3]) or lower.

While description has been made in connection with the specific exemplary embodiment and the examples thereof, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the present invention. It is aimed, therefore, to cover in the appended claims all such changes and modifications falling within the true spirit and scope of the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   100 mold 

1. A hot press forming method comprising: heating a steel plate so that an average temperature of the steel plate becomes Ac3 point or higher; executing a press forming on the steel plate whose average temperature is so set by use of a mold; cooling the steel plate during press forming; removing a formed product from the mold, while the average temperature of the formed product stays within a temperature range from 322° C. or higher to 510° C. or lower; and cooling the formed product further.
 2. The method according to claim 1, further comprising: setting the average temperature of the steel plate to 366° C. or higher, when the steel plate is removed from the mold.
 3. The method according to claim 1, further comprising: setting the average temperature of the steel plate to 451° C. or lower, when the steel plate is removed from the mold.
 4. The method according to claim 1, further comprising: setting the average temperature of the steel plate to 451° C. or lower, when the steel plate is removed from the mold. 